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Failure Analysis On A Super Duplex Stainless Steel Tube Material Before And After Slow Strain Rate Testing In Sour Conditions - Part 1: SCC Susceptibility

Failure Analyses On A Super Duplex Stainless Steel Tube Material Before And After Slow Strain Rate Testing In Sour Conditions – Part 1: SCC Susceptibility
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Standards NACE(1) MR0175 / ISO(2) 15156-31 have temperature-H2S-Cl-pH limits for safely using duplex stainless steels based on their Pitting Resistance Equivalent Number (PREN) of subcategory such as duplex stainless steel (DSS, PREN 30-40, Mo>1.5%) and super duplex stainless steel (SDSS, PREN 4045), temperature-pH domain of stress corrosion cracking (SCC) resistance can be used for material evaluation and provided by using slow strain rate testing (SSRT) screening in sour testing conditions2. 

Product Number: 51322-17998-SG
Author: Wenle He, Raveendra Siriki, Jerry Lindkvist
Publication Date: 2022
Industries: Corrosion Monitoring and Control , Coatings
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$20.00
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For safely using duplex stainless steel and super duplex stainless steel, temperature-pH domains of stress corrosion cracking (SCC) resistance were provided by using slow strain rate testing (SSRT) screening in sour conditions in an earlier study.  To follow-up the study, a failure analysis has been performed on the tube material of UNS S32750 and the specimens after the SSRT in a sour environment in pH 2.8, pH 3.5 at 80 °C. Scanning electron microscope (SEM) equipped by energy-dispersive X-ray spectrometer (EDS) analysis and Electron backscatter diffraction (EBSD) measurements have been used for the investigation on specimen surface, fracture surface and cross-sections of the SSRT specimens. Initiation of selective dissolution (SD) and cracking has been identified. EBSD reveals that cracking initiated in ferrite with enhanced strain/deformation in frontend of selectively dissolved austenite, where possible SCC mechanism seems to be hydrogen embrittlement. It observed that SD stopped in the brittle-ductile transition area in the fracture surface, which probably attributed to repassivation of the material in the condition of pH 3.5. Combined SSRT and analysis methods by SEM and EBSD showed a good way for the material evaluation before decision making for the materials selections and supplementary long-term exposures. 

For safely using duplex stainless steel and super duplex stainless steel, temperature-pH domains of stress corrosion cracking (SCC) resistance were provided by using slow strain rate testing (SSRT) screening in sour conditions in an earlier study.  To follow-up the study, a failure analysis has been performed on the tube material of UNS S32750 and the specimens after the SSRT in a sour environment in pH 2.8, pH 3.5 at 80 °C. Scanning electron microscope (SEM) equipped by energy-dispersive X-ray spectrometer (EDS) analysis and Electron backscatter diffraction (EBSD) measurements have been used for the investigation on specimen surface, fracture surface and cross-sections of the SSRT specimens. Initiation of selective dissolution (SD) and cracking has been identified. EBSD reveals that cracking initiated in ferrite with enhanced strain/deformation in frontend of selectively dissolved austenite, where possible SCC mechanism seems to be hydrogen embrittlement. It observed that SD stopped in the brittle-ductile transition area in the fracture surface, which probably attributed to repassivation of the material in the condition of pH 3.5. Combined SSRT and analysis methods by SEM and EBSD showed a good way for the material evaluation before decision making for the materials selections and supplementary long-term exposures. 

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Product Number: 51322-17613-SG
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Duplex stainless steels (DSSs) are based on the Fe-Cr-Ni system and are constituted of 30 to 70 % ferrite and austenite. They combine high tensile strength, good toughness, weldability, and excellent corrosion resistance including stress-corrosion cracking and resistance to localized corrosion.1-3 DSSs can be classified according to the Pitting Resistance Equivalent Number (PREN = Cr + 3.3 Mo + 16 N) in lean duplex (PREN= 22-27), standard (PREN = 28-38), super duplex (PREN = 38-45) and hyperduplex (PREN > 45).
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